[firebase_hosting] Wasm is not used in Flutter Web App

[Kuhlemann]

Problem

Wasm is not used when deploying to Firebase Hosting and I do not understand why.

Analysis

Deploying the same flutter code locally works with Wasm-active.
I checked all requirements which I found at: https://docs.flutter.dev/platform-integration/web/wasm

Steps to reproduce

1. I use Flutter stable channel on version 3.24.0
2. I use the simplest possible code to reproduce (see sample below)
3. To make sure there is no problem inside the index.html I deleted the "web"-folder and recreated it with: flutter create . --platforms=web
4. flutter build web --wasm compiles successfully: "✓ Built build/web"
5. Directly after that I start a local server in my "build/web"-folder with dhttp for testing purpose: dhttpd '--headers=Cross-Origin-Embedder-Policy=credentialless;Cross-Origin-Opener-Policy=same-origin'
6. I use Chrome version 126 to open localhost:8080 and kIsWasm is true.
7. After that I deploy with firebase deploy and kIsWasm is now false in the online version of my app.

What am I missing (wrong CORS configuration for example)?

void main() {
  runApp(
    MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('WebAssembly-Status: $kIsWasm!'),
        ),
        body: Center(
          child: Text('Welcome to Flutter WebAssembly!'),
        ),
      ),
    ),
  );
}

My firebase.json in my project folder looks like this:

{
  "hosting": {
    "source": ".",
    "ignore": [
      "firebase.json",
      "**/.*",
      "**/node_modules/**"
    ],
    "frameworksBackend": {
      "region": "europe-west1"
    },
    "rewrites": [ {
      "source": "**",
      "destination": "/index.html"
    } ],
    "headers": [
        {
           "source": "**/*",
            "headers": [
                {
                    "key": "Cross-Origin-Embedder-Policy",
                    "value": "require-corp"
                },
                {
                    "key": "Cross-Origin-Opener-Policy",
                    "value": "same-origin"
                }
            ]
        }
    ]    
  }
}

[shrawank039]

When you run the firebase deploy cmd in flutter. It runs flutter build web --release automatically before deployment. So as a result it rebuilds the project in release mode without the wasm support.

[shrawank039]

to avoid the auto build while firebase deploy you should set the public path in your firebase.json file.

{
  "hosting": {
    "public": "build/web",
    "ignore": [
      "firebase.json",
      "**/.*",
      "**/node_modules/**"
    ],
  }
}

changing "source": "." to "public": "build/web" the automatic build will stop on firebase deploy